To date, the developed S or N type negative differential resistance (NDR) devices in the world are applied extensively in the high-speed switches or high frequency oscillator. meanwhile, a real space transfer device is designed on the basis of the principle that by using voltage, electrons move at high speed to produce energy. When the energy exceeds a threshold value of the potential energy in the epitaxy, the electrons conduct (transfer) vertically to the adjacent epitaxy with the scattering effect of the atomic lattice. The change of electron location is called real space transfer (RST) . The applications of the real space transfer devices that use hot electron emission include microwave generations, high-speed logical circuits, programmable random access memory elements and photo emission devices have high potential. The major operation modes of real space transfer devices are the three terminal type negative resistance field-effect transistor (NERFET) and the charge injection transistor (CHINT).
However, most of the research relating to real space transfer over-emphasize the heterostructure of the AlGaAs/GaAs, AlGaAs/InGaAs/GaAs and InAlAs/InGaAs/InP systems. From the views of this RST structure in different systems, the Al component included structures may produce the deep energy level complex center (DX centers) which will affect the device characteristics. Another similar case in the structure of the AlGaAs and AlGaAs/InGaAs/GaAs, since the mobility of the AlGaAs barrier is much lower than that of GaAs barrier structure, the valley current range of the AlGaAs is narrower. In addition, since the doped efficiency of the AlGaAs is lower, attempting to increase the peak current value is not easy.
A heterostructure that relates to the high performance .delta.-doped GaAs/InGaAs structure is proposed by C. Y. Chang et al. (1991) in the Japanese Journal of Applied Physics, pages 1158-1165. A .delta.-doped layer is a maximally-concentrated doped layer with the distribution of heterogeneity (impurity or foreign matter) to be restricted within a distributed single atomic layer in semiconductor materials. Herein, the heterostructure field-effect transistor system (HFET) which is prepared with GaAs/InGaAs materials reveals an effective improvement of the deficit that is mentioned above. In order to effectively improve the deficits and promote the real-space transfer characteristics, four types of multiple-function transistors, which simultaneously possess real space transfer and field effect transistor (FET) behavior, are proposed in this invention.